SKA pathfinders reveal the messy eating habits of black holes

BY DR HILARY KAY (THE UNIVERSITY OF MANCHESTER – UK SKA)

An international team of researchers has used ultra-sensitive radio observations with the UK’s e-MERLIN National Facility and the European VLBI Network (EVN), both SKA pathfinders, to shed light on the eating habits of supermassive black holes. Their conclusion: black holes are rather messy eaters!

Active galaxies have been studied extensively by astronomers for more than six decades. At their heart lies a supermassive black hole, which swallows material from its close surroundings, causing the emission of radio, infrared, ultraviolet and X-ray radiation. However, not all supermassive black holes consume, or accrete, material in the same fashion.

Artist’s impression of a galaxy with an active nucleus, a supermassive black hole in the centre. CREDIT: ESA/C. Carreau

The team discovered that distant active galaxies exhibit a wide variety of behaviours. Whilst some accrete material at a rapid rate, others accrete at a slower, steadier rate, with the remainder not accreting material at all. This variation in behaviour was also seen in the relationship between accretion and star formation in the host galaxy. Whilst the accretion phase sometimes occurred simultaneously with a period of star formation, this was not true in the majority of cases. If star formation is ongoing, activity in the nucleus is difficult to detect. In addition, the nuclear accretion process was not found to consistently generate radio jets, regardless of the accretion rate.

These high-sensitivity observations illustrate the importance of radio observations in understanding the processes at work in the centre of galaxies. “This means the SKA telescopes and their precursors such as MeerKAT and ASKAP will play a crucial role in unravelling the eating habits of supermassive black holes in unprecedented detail,” says principal investigator Dr Jack Radcliffe of the University of Pretoria, South Africa and The University of Manchester, UK.

“We are getting more and more indications that all galaxies have enormously massive black holes in their centres. Of course, these must have grown to their current mass,” adds co-author Prof. Peter Barthel of the University of Groningen in the Netherlands. “It seems that, thanks to our observations, we now have these growth processes in view and are slowly but surely starting to understand them.”

These results have been published in the Astronomy & Astrophysics journal and are available on the arXiv: https://arxiv.org/abs/2104.04519 and https://arxiv.org/abs/2103.08575